NASHVILLE, Tenn. -- The first time Margaret Ann Horn broke out in an odd rash, she blamed it on the sumac she had cleared at her farm in Leipers Fork.

Then the same rash from last summer came back in December, worse than before.

"I had to go to the emergency room for the first time in my life," Horn said. "I had some pretty severe swelling. I had a rash on my chin. I had to have IVs and all that stuff."

When an internist diagnosed her with an alpha-gal allergy, she wondered, "Alpha what?"

Red meat — specifically a type of sugar called alpha-gal present in red meat — triggered the allergic reaction, Horn learned. The diagnosis puzzled her because she had eaten steaks, barbecue and hamburgers her whole life with no problems.

Now she has had to completely change her diet because of a tick bite.

Bites from lone star ticks — reddish-brown ticks that come out when spring arrives — are causing people across the Southeast to develop allergies to red meat. Clinics with the Vanderbilt Asthma Sinus and Allergy Program are diagnosing one or more cases a week, said Dr. Robert Valet, an allergist.

Horn didn't even know she had been bitten by a tick. Owning a 50-acre farm and being an avid hiker, she said she's sure she has come into contact with ticks.

"I have been very much a carnivore," Horn said. "Now, I'm a reluctant vegan."

Doctors did not even know tick bites could trigger the food allergy until researchers at the University of Virginia made the connection. Dr. Thomas Platts-Mills led the team that noticed the trend of people developing the allergy later in life. After reporting this finding in 2009, his team worked with other research institutions and concluded in 2011 that tick bites were the cause.

A blood test is used to confirm an alpha-gal allergy.

"This is one of the most important food allergies in the Southeast, even though we've only known about this for a very short time," Valet said. "Once it develops, there is not a way to make this stop, so we really encourage people to be careful about exposure."

Reactions include hives, swelling, vomiting, diarrhea, breathing difficulties and drops in blood pressure. The severity can vary among patients, he said.

Some patients are especially sensitive to exposure, having to avoid milk as well as foods cooked on the same grill as red meat.

Research has shown that people with the allergy undergo a 20-fold or higher increase in antibodies to alpha-gal after being bitten by the ticks.

Horn has scheduled an April appointment with Platts-Mills in hopes of learning about research on therapies to address her red meat allergy. She said she has to avoid vegetable dishes flavored with meats as well as gelatin products, such as the gel caps on diet supplements.

And she still craves the foods she loves.

"I'm not a big chicken eater, and I'm not a huge fish eater," she said. "My favorite is good rare filet, a beef tenderloin or barbecued ribs from Memphis."